1. Field of the Invention
The invention is directed to a method and apparatus for monitoring the output of an ion chamber type neutron detector.
2. Description of the Prior Art
The invention relates to ion chamber type neutron detectors and systems thereof that are used to measure neutron flux in a nuclear reactor core. An example of an in-core detector system for measuring and monitoring the neutron flux in a nuclear reactor core with which the present invention may be employed is shown by G. R. Parkos et al., in U.S. Pat. No. 3,565,650.
Ion chamber type neutron detectors are well known and are shown, for example, by L. R. Boyd et al. in U.S. Pat. No. 3,043,954. Usually, such chambers comprise a pair of spaced electrodes electrically insulated from one another with a neutron sensitive material and an ionizable gas therebetween. For example, in a fission type ion chamber or fission chamber, the neutron sensitive material is a material such as U-235, which is fissionable by neutrons. As incident neutrons induce fissions of the uranium in the chamber the resultant fission products ionize the gas in proportion to the magnitude of the neutron flux in the chamber. Other types of neutron sensitive ion chambers employ a neutron sensitive material such as boron trifluoride in gaseous form. When a direct current voltage is applied across the electrodes of these ion chambers, an output current is created which is proportional to the amount of ionization and hence proportional to the neutron flux in the chamber.
It is well known that the neutron flux in the fission chamber may be determined by either measuring the average current flowing through the chamber to generate a signal, normally referred to as the DC signal, representative of the direct current flowing through the chamber, or by measuring the mean-square alternating current in the chamber in a suitable range of frequencies to generate a signal, normally referred to as the AC signal, representative of the alternating current flowing through the chamber. Either of these methods generate a signal which is used as a measure of the neutron flux in the chamber. Currently, in boiling water reactors, the direct current signal is used as a measure of neutron flux in the power range of the reactor and the alternating current signal is used as a measure of neutron flux at lower power levels.
As neutron detectors, fission chambers have the advantages of good sensitivity, adequate life and prompt response to changes in neutron flux. However, their response tends to be nonlinear and the output current versus neutron flux for any given chamber is not predictable with exactness. Furthermore, during use, the chambers must be recalibrated rather frequently because of loss of sensitivity due to burn-up of the neutron sensitive material or due to a change in the density of the ionizable gas in the chamber. In general, the operation of such fission chambers is easily impaired and malfunctions of various kinds can cause changes in sensitivity, the presence and magnitude of which may remain undetected until recalibration.
One of the weakest parts of a fission chamber is the seal between the chamber and the connecting cable. This seal contains the gas in the chamber and maintains a constant gas density in the chamber. When this seal fails, gas can flow either out of the chamber into the cable or out of the cable into the chamber depending on the gas pressures in those two regions at the time of the seal failure. In either case, the sensitivity of the chamber changes and the alternating current and direct current signals generated by the chamber become erroneous measures of neutron flux. Since this gas density change can occur over a period of time that can vary from a few minutes to several days depending on the degree of the failure, the erroneous reading may not be detected. In addition, if the erroneous reading is detected there is no way in which the size of the error can be determined other than by recalibrating the fission chamber. Thus, a need exists for a system which will detect a change in density of the gas in the ion chamber and measure the size of the error produced so that the output of the detector may be corrected automatically.
Another problem encountered with fission chamber neutron detectors is that gamma radiation will also ionize the gas in the chamber and produce a direct current signal proportional to gamma radiation in the chamber. There is no way to distinguish the neutron-produced portion from the gamma-produced portion of the direct current signal generated by the chamber. Thus, when the direct current signal is used as a measure of neutron flux, which is the method presently used in the power range of boiling water reactors, the fission chamber is considered to have reached end-of-life when the neutron produced current falls below a certain predetermined fraction of the total chamber current. However, there is presently no way to determine this event since the gamma exposure rate in the vicinity of the detector in the core of the reactor is not known and cannot accurately be measured. Thus, when the direct current signal is used as a measure of neutron flux, a need exists for a system which will measure the fraction of detector current that is being produced by neutron irradiation so that the end-of-life of the detector can be predicted.
Another problem encountered with fission chamber neutron detectors is that the chamber response is non-linear. That is, its output current is not strictly proportional to the neutron flux in the chamber. This non-linearity is due to reactor power induced temperature variations in the detector which result in power dependent variations in the gas density of the active volume of the detector. This results in detector sensitivity that is power dependent and hence a detector response that is non-linear. Since it is impossible to accurately measure neutron flux in the vicinity of a detector in the core of a nuclear reactor, the non-linearity cannot be determined and corrected in the conventional manner of measuring chamber output versus neutron flux in the range of neutron flux in which the chamber is to be used. Nevertheless, it is important to determine the detector non-linearity since the maximum power level at which a modern high power density reactor can be operated is a function of the non-linearity of its in-core detectors. Thus, a need exists for a system that determines the non-linearity of an ion chamber type neutron detector at various power levels.
Thus, it is an object of the present invention to provide a monitor for rapidly detecting a gas leak or a change in gas density in a fission chamber neutron detector.
It is another object of the invention to provide a monitor for correcting the output of a fission chamber neutron detector for changes in the density of the gas in the chamber.
It is another object of the invention to provide a monitor for measuring the neutron-produced fraction of the direct current signal generated in a fission chamber neutron detector so that the end-of-life of the detector may be predicted when the direct current signal is used as a measure of neutron flux.
It is another object of the invention to provide a monitor for determining the non-linearity of a fission chamber neutron detector at various power levels.
It is another object of the invention to substantially reduce or eliminate the number of times a fission chamber neutron detector needs to be recalibrated.